CiviUnitTestCase - Fix miscoordination with CiviEnvBuilder

CiviUnitTestCase - Fix miscoordination with CiviEnvBuilder

Background
----------

Recall that `CiviUnitTestCase` and `CiviEnvBuilder` are two different
implementations of a similar concept.

* `Concept`: If two tests use the same baseline DB/environment, and if they
  preserve the baseline, then you don't need to reset everything in between
  tests.  But if they change, then you need to reset.

* `CiviUnitTestCase`: The older rendition.  Tightly coupled.  Only allows
  one specific baseline.  Mingled with a lot of unrelated functionality in
  `CiviUnitTestCase`.

* `CiviEnvBuilder`: The newer rendition.  Loosely coupled.  Allows different
  baselines.  Can be mixed into any plain-old `TestCase`.  Used by
  `Civi\Test::headless()` and `Civi\Test::e2e()`.

Problem Scenario
----------------

Suppose you have a mix of different tests running with the same DB -- e.g.

1. Run some tests based on `CiviEnvBuilder`
2. Run some tests based on `CiviUnitTestCase`
3. Run some tests based on `CiviEnvBuilder`

This wasn't originally anticipated, but it can happen -- either because
the test-suite is large+mixed, or because a developer is manually
running specific tests (which happen to be written differently).

The problem goes like this:

1. Run some tests based on `CiviEnvBuilder`
    * This resets the DB and also stores a DB signature. ("Here is how
      we setup the DB...")
2. Run some tests based on `CiviUnitTestCase`
    * This resets the DB, but leaves the old DB signature in place.
3. Run some tests based on `CiviEnvBuilder`
    * This sees the old DB signature and wrongly concludes that we
      still have the DB from step (1).

Solution
--------

Whenever one resets the DB, it should update the DB signature.